Optical systems for the gathering of infrared radiation for passive infrared intrusion detection devices are well known in the art. In particular, infrared intrusion detection devices which are mounted on ceilings and can detect intruders within a 360.degree. degree field of view are well known in the art.
However, heretofore, a 360.degree. field of view optical system for passive infrared intrusion detection has used either a combination of two mirrors and a fresnel lens or two mirrors to receive the infrared radiation. The use of mirrors requires that the mirrors have superior surface finish and excellent reflection coating properties. Such mirrors with excellent coated qualities necessarily mean that they are expensive.
Another prior art ceiling mounted device for the collection of infrared radiation from a 360.degree. field of view uses a plurality of fresnel lenses. In such a device, the plurality of fresnel lenses collect infrared radiation and focus them at a focal point where a single sensor is located. The problem with the use of fresnel lenses alone is that there is no long range detection capability. In addition, this prior art device has a "dead" zone wherein the device is unable to detect intruders in that zone or is able to detect intruders only when they are very close to the detector. This has limited the use of such a device.